Original subtitles

PROFESSOR BRIAN COX: Ihis creature is a wonder of nature.

Its biology is hard-wired to the heavens.

It has exguisitely sensitive eye

that locks onto the Sun

and allows it to navigate its way across the face of the planet.

In a sense, it has an instinctive understandıing

of its place in the solar system.

A tiny insect brain

Joined to the movements of the Sun and the planets.

Ihis connection steers the monarch and millions of its brethren

as they make öne of the longest migrations

of any butterfly species.

They're heading these trees, known locally as the oyamel

or sacred furs.

And some of the butterflies began their journey over 4,000 kilometres away,

that's 2,500 miles,

up here in the north-eastern United States and Canada.

And over the autumn and the winter

they've migrated south, across the United States,

and arrived here, in Central Mexico.

Incredibly, no butterfly has ever learnt this route.

It can't have, because İit takes at least three generations

to make the round trip.

Instead, the homing instinct is carried on a river of genetic information

that flows through each butterfly.

The allure of this place to the butterflies,

this sense of belonging,

is a deep feeling we all share.

We even have a word for it.

Home.

Every living thing that we know to exist is found on this one rock.

So what is it about our planet

that makes it such a rich, colourful, living world?

I want to show you why our world is the only habitable planet we know of

anywhere in the universe.

And the answer depends on the presence of a handful of precious ingredients

that make our world a home.

(EBAGLES SCREAMING)

ASTRONAUT (ON RADIO): / tfhe begğinning, God created the heaven and the Farth.

And the Earth was without form and void.

And darkness was upon the face of the deep.

Home is such a...an evocative word.

I mean, it will mean something to you.

The place you went to school, the place you lived,

the place where your kids had their first Christmas.

But in a scientific sense, what does it mean?

İt means that the ingredients are there for you to live.

An atmosphere, food, water.

You need the temperature to be right.

Home is the place that has the things you need

for your biology and chemistry to work.

It's no less evocative for that.

-(MAN GRUNTS) -(HORSE NEIGHING)

(COW MOOING)

This is Mexico,

a country rich in the ingredients that set our world apart.

İIt's not a bad place to come,

because with that 126 of the land surface area of our planet,

it's home to 1296 of the species.

There are 26,000 plant species here.

There are 700 species of reptiles

and 400 species of mammals.

İt's also been home to some of the world's great civilisations.

The Maya built their temples out there in the forest here

for thousands and thousands of years.

(MAN SINGING IN SPANISH)

Mexico is bursting with life.

And if you know where to look,

hidden inside these creatures

are clues that tell how this planet became their home.

First stop is in the south-east of the country,

an area covered in thick jungle.

The Yucatan's a strip of essentially pure limestone

that separates the Caribbean from the Gulf of Mexico,

and it's got all the ingredients you might think you need

for a rich and diverse ecosystem.

The tropical sun warms the forest,

delivering precious energy to each and every leaf.

Öxygen escapes from the plants and trees,

which is breathed in by the forest animals.

And, where they can,

each of them draws deeply from the regiron's hidden water supply.

But there are some of the ingredients you need

to grow this tropical forest that are far more important than others.

You might think that this place would be awash with water.

And it does rain a lot and it's incredibly humid.

Bukt, actually, there are no surface rivers at all on the Yucatan Peninsula

because the water just seeps into the porous limestone.

That's where these things come in.

These are cenotes,

they're caverns dissolved out of the limestone by the rain.

And they collect water.

They play a vital role in the ecosystem.

I mean, the forest changes when you get around a cenote.

-Just listen to that. -(FROGS CROAKING)

Those are frogs,

and you don't hear those frogs anywhere else in the forest,

Just around the cenotes.

The cenotes are flooded caves

that have been cut off from the outside world

for thousands of years.

Lilies, troglodytic fish, even the occasional turtle,

all thrive around the openings of these freshwater welils.

As I head deeper into the cave,

the temperature drops and the light fades.

Öne by one, the ingredients I depend upon

begin to disappear.

Yet even here, far from the soil and air,

strangely-coloured algae still find a home in the water.

If there's öone thing that unites every form oflife in this cenote,

in fact, every form of life out there in the forest,

in fact, every form of life we've ever discovered

anywhere on planet EFarth,

is that it has to be wet.

Only on our home does water run freely

between the skies, oceans, rivers and on, into every living thing.

(SPEAKING IN SPANISH)

To understand why life and water are so intertwined,

we need to look a little deeper

into öone of the strangest substances we know.

(SPEAKING IN SPANISH)

Now, I may be a bit of a middle-aged academic,

but I can still do the odd experiment every now and again.

So what I'm doing is I'm charging up this Perspex rod.

So, giving it an electric charge by rubbing it on a fleece.

Now, watch what happens

when I put the rod next to a stream of water.

You see that?

Look at that.

The electrical field, the electric charge, is bending the water towards İt.

Now, the reason for that,

the reason that water behaves in that way

when it's passing through an electric field,

is exactly the same reason that it is vital for all life on Earth.

Water is polar molecule,

which means it responds to electric charge.

Its polarity comes about because of the structure

of water molecules themselves.

Now, water is H20,

two hydrogens and one oxygen atom bound together.

So, two hydrogen atoms approach oxygen.

Oxygen's got a cloud of eight electrons around İt.

So when the hydrogens come in,

then what happens is the electrons get dragged over here,

around the oxygen.

So you end up with the electron cloud around here,

and, to some extent,

pretty isolated, positively-charged protons out here.

So you get a net positive charge over here,

and the electron cloud with its negative charge over here.

So you get what's called a polar molecule.

And that's why when you bring a charged Perspex rod

close to water molecules, they bend towards it.

Water's polar nature means

that, although its molecules are simple,

together they form a subtle, endlessliy-complex ligurd.

A home in which öne tiny creature thrives.

There he is. Look at that.

That is a pond skater,

a predator that floats on the surface of the water

and actually uses the surface of the water

to sense its prey.

Pond skaters are vicious predators

that live for most of their lives on the surface.

Tiny hairs on their legs provide a large area

that spreads their weight.

Their middle legs thrust them forward.

Hind legs are employed to steer.

They're so well adapted to life in this flat world

that they even sense their sexual partners

through tiny vibrations in the water's surface.

The reason it can do that

is the result of a complex interaction

between adaptions in the animal itself

and the physics, the chemistry, of the surface water.

Water molecules are polar,

and that means that water molecules themselves

can bond together.

So you can get hydrogen, with its slight positive charge

getting close to the oxygen of another water molecule,

with its slight negative charge,

and bonding to İt.

And you can build up duite large, in fact, very large structures

in liguid water.

This is what gives water its unigue ability

to form a surface habitat for the pond skaters.

Clumps of H20 stick together,

keeping the surface under tension,

forming a chorus of water molecules,

alljoined together by hydrogen bonds.

Then a pond skater comes along

and it puts its legs, or its

dangly things into the water

and pushes it down,

bends the surface of the water.

Now, the water doesn't like that because a bend in the water

is increasing its surface area,

it's increasing its energy,

it's making it harder for all the molecules

to bond together with the hydrogen bonds.

So they try to push back.

They exert a force on the pond skater's leg

because they want to bond as much as they can.

And that's how pond skaters stay on the surface of the water.

Hydrogen bonds do far more

than just give the pond skaters a place to live.

They're fundamental to all life.

I've heard it said

that we won't truly understand biology until we understand water.

These are very thin tubes of glass.

They're about, what, a millimetre in diameter.

Ifl dip one into the surface of this river,

you see that the water just climbs up the tube.

İt pulls itself up, guite literally,

against the force of gravity.

Now, in trees there are tubes

which are about half the diameter of this,

perhaps about half a millimetre or even less.

And they are called xylem.

And they allow the tree to lift water up through the root system,

because the water molecules strongly attract each other

and are strongly attracted to the sides of the tubes.

So when you look at trees like that,

which are very high, and you ask yourself the guestion,

"How do they get the water up from their roots to the top of the tree?"

A big part of that is capillary action,

which is down to the polar nature of water.

Öne of water's most important gualities

is its ability to dissolve and carry

all manner of substances around the living world.

Because its molecules are very small and polar,

water İs a tremendously effective solvent.

Those molecules can get in amongst other substances,

salts and sugars, for example,

and disperse them, if you like, in that sea of hydrogen bonds.

Within every one of us,

Water is constantiy flowing around each and every celi.

Blood plasma is over 9096 water,

and in it are dissolved everything I need to live.

Oxygen, the nutrients from food, everything,

distributed around my body in rivers of water.

We live on a beautiful, blue anomaly of a world.

The only planet we know

With a surface drenched in liguid water.

The story of how each drop ended up here has been hard to fathom.

Iargely, because it happened so long ago,

there's very little direct evidence.

But back in the Yucatan jungle,

clues to how it turned up can still be found.

Every civilisation on the Yucatan,

be it the modern Mexicans or the Mayans,

had to get their water from those deep wells,

the cenotes.

And I've got a map, a completely unbiased map

of the largest cenote. İt's here,

which I'm going to overlay on the Yucatan.

Look at that. They lie in a perfect arc,

centred around a very particular village

which is there. And it's called Chicxulub.

Now, to a geologist,

there are very few natural events

that can create a structure,

such a perfect arc as that.

All the evidence points to just one explanation.

You're looking at what's left

of a gigantic asteroid strike.

Öne that wiped out three guarters of all plant and animal species

when it hit the Farth 65 million years ago.

You may think that impacts from space are a thing of the past,

a thing that only happened to dinosaurs.

But that's not true either.

About 55 million kilograms of rock hits the Earth every year,

and around 296 of that is water.

This hints that at least some of Farth's water arrived from space.

Iate in 2010,

these glimpses of Comet Hartley 2 arrived back on Farth.

They were sent by NASA's Deep Impact probe.

From its surface, dust and ice spray into space.

Analysis of this water

found it had a very similar mixture of isotopes

to the water in our own öoceans.

Ihis was first firm evidence

that icy comets must have contributed to the formation of our world's öoceans.

Farth began life as a molten hell.

Its internal heat drove off any trace of moisture.

But, soon, the planet cooled

-and the first clouds grew. -(THUNDER RUMBLING)

Then, 4.2 billion years ago,

a deluge, the like of which

the solar system had never seen before or since, rained down.

And, ağain, thanks to those hydrogen bonds,

Water's borling porint is high enough

to have allowed it to remain on the surface of the Farth

to the present day.

So from duite early in its history,

our home has been able to hang on to this most vital of ingredients.

But to trace the origin of the next ingredient

you have to look beyond our planet.

To our nearest star.

And the rays of light it sends our way.

This is the train from Los Mochis to Chihuahua

which inexplicably leaves at 6:00 a.m. in the morning.

The local name for this area in all the gulde books

is "The Land of Turtles".

Beautifully romantic name for this place on the Sea of Cortez.

But, actually, we just found out that it's probably more likely

to have been called "The Land of Spinach-type Vegetables".

So we're going from "The Land of Spinach-type Vegetables"

to Chihuahua,

which is "The Land of Very Small Dogs".

One of the great railway Journeys of the world.

(TRAIN WHISTLE BLOWING)

Almost all life depends on the energy

that the Sun sends our way.

But the Sun is a far from benevolent companion,

because its radiant ralin can be as dangerous

as it is nourishing.

We're still round about sea level now,

and the Sun is guite low in the sky.

It's about 7:00 a.m., so it's not been up long.

And I'm going to measure the amount of UV radiation

falling on every sguare centimetre with this,

a digital ultraviolet radiometer.

At the moment,

it says there's about 22 microwatts per sguare centimetre

falling on my skin.

But as we climb in altitude,

then that UVB light

is gonna have to travel through less and less of the atmosphere,

so less of it's going to be absorbed.

And, sure enougğh, as the miles pass by

and we head into the mountainous interior,

the meter readings start to go up.

(TRAIN WHISTLE BLOWING)

Now it's about 10:00 in the morning,

so the Sun's significantly higher in the sky.

The train's also climbed guite a bit in altitude.

So now,

we're getting nearly 250 microwatts per sguare centimetre.

So that's a factor of 10 higher,

and that's just because the UVB has had significantly less atmosphere

to travel through from the top of the Earth's atmosphere down to me.

That's more than enougğh to burn unprotected skin in just a few minutes.

And that's because what arrives from the Sun

is far more than just the stuff we can see.

Beyond the visible,

in the higher energy part of the spectrum,

there's ultraviolet light, particularly UVB,

which does get through the Earth's atmosphere

and gets to the surface.

Now, UVB can be beneficial to life.

We use it to produce vitamin D, for example.

But because it's higher energy,

it can also be extremely damaging.

It can damage DNA, it can burn our skin as well as give us a suntan.

And, of course, ultimately, it can give us skin cancer.

(TRAIN WHISTLE BLOWING)

Now, if ultraviolet light is a problem for life on Earth to deal with today,

then the physicist might raise

an interesting problem for the biologist.

Because we know that three and half billion years ago,

when life on Earth began,

although the Sun was much dimmer

in the visible part of the spectrum,

it was significantly brighter in the ultraviolet.

The young Sun seems like a paradox.

İt was fainter to the eye,

perhaps 3096 less bright than the Sun we enjoy today,

yet rich in deadiy ultraviolet.

Inside, the core was spinning much faster,

which created more electromagnetic heating

of the plasma on its surface.

And this plasma emitted more energy, not in the lower visible freguencies,

but in the higher freguencies.

Like X-rays.

And ultraviolet.

İt seems as if just as life Was getting settled on its wet home,

the faint young Sun was making it tough to survive near the surface.

This is the top of Copper Canyon,

so the summit of the railway journey. İt's about 2,200 metres,

which is about, what, somewhere between 7-8, 000 feet.

So I'll take a UV reading of the Sun.

Yeah, it's actually reading about 260 now.

Now, if you remember, at midday, down at sea level

we were getting readings around 260.

So although the Sun has dropped in the sky,

so the sunlight and the UV are coming through much more atmosphere,

that's been compensated for by the thinness of the air up here.

I'm getting more UV now

than I would have been at the same time of day at sea level.

It's hard to be sure,

but we think that it's these kinds of radiation levels

that early life had to deal with.

Because back then the Sun's ultraviolet output

Was significantly stronger.

So I think it is fair to say that that could have posed a significant threat

to the development of early life on Earth.

(HORSE NEIGHS)

Today, life has painted the surface of our home

in all the colours of the rainbow.

From greens to blues, reds to yellows,

oranges and violets.

And the origin of all life's hues can be traced back

to the way it interacts with sunliğht.

I'm a particle physicist, so I'm allowed to think of everything

in terms of the interactions of particles.

So I would picture the light from the Sun as being,

really, a rain of particles, photons they're called,

particles of light of different energies,

raining down on the surface of the Earth.

The blue ones are the highest energy photons.

The red ones are the lowest energy photons.

And all the colours of the rainbow in the middle

are just simply photons with different energies.

(SPEAKING IN SPANISH)

Ah, thank you.

Wow.

So, for this, a chilli salsa, which I see as red,

then there are pigment molecules in there

that are absorbing the blue photons, the blue light from the Sun.

The red ones it doesn't interact with, and so they bounce back into my eye

and that's why I see İt as red.

Same with the green chilli. But in this case,

the red photons are interacting, they're doing something,

they're talking to the pigments in here,

and what I'm seeing are the green photons

and some of the blue photons coming into my eye,

mixing up and allowing me to see that as green.

(DOG WHIMPERS)

Pigmenits bring colour to the world.

A planet painted by genes,

honed by billions of years of evolution.

Some colours warn of danger...

This stuff's on fire, I tell you.

Or attract pollinators.

Pigments are öone of the ways that life has evolved

to take on the Sun's powerful ultraviolet light.

This little guy is called a bombardier beetle.

If I just grab him...

His name comes from his unigue defence mechanism.

He produces two chemicals,

oöne of them you might have heard of, hydrogen peroxide.

And the other one is something called hydroguinone.

And when you scare him,

both those chemicals are injected into a little chamber in his body.

Which raises the temperature to the boiling point of water,

and increases the pressure,

sguirting a hot, noxious chemical Ousvt of its rear.

What a clever way to defend yourself.

But this is just öone of the ways this character uses chemistry

to increase its chances of survival.

Now, the bombardier beetle and me

and, in fact, everything living thing you can see,

are exposed to the same threat here on the high plains of Mexico.

İt's the high-eneray, ultraviolet photons

raining down on this landscape from the Sun.

If they hit DNA, in my skin, for example,

then they'll damage the DNA.

So that's got to be prevented.

Me and my friend the beetle

have both reached the same solution.

You see, the beetle is brown and black.

My skin, oönce exposed to the Sun, is going brown,

because I'm producing a pigment called melanin.

And so is the beetle.

Now, melanin is a very simple molecule,

it's just a ring of carbon atoms with a few extra bits bolted on.

But its sea of electrons behaves in a very specific way.

When a high-energy ultraviolet photon from the Sun

hits one of those electrons,

it very, very gulckly dissipates that energy.

That potentially threatening photon has been absorbed

and all its energy has been dissipated away as heat.

Melanin is so efficient that over 99.996

of the harmful ultraviolet radiation is absorbed.

So melanin is protecting both my skin

and my friend the bombardier beetle

from the potentially harmful effects of the Sun.

(TRAIN WHISTLE SOUNDS)

From the starkt, life had to evolve strategies

for coping with the energetic young Sun.

Now, life is nothing if not resourceful,

and pigments are the way that living things interact

with the radiation from the Sun.

So why just use them to dissipate energy, to protect?

Why not use them to harness the energy for its own ends?

Well, that's exactly what life did.

And, in doing so, it transformed our planet

by introducing a Wonderful new ingredient.

Farth has an atmosphere unlike any other planet we know of in the universe.

Only in the air in our world do fires burn.

Only in our world has a gas been released

which allowed complex life to evolve.

What makes our home unigue

is its oxygen-rich atmosphere.

Deep in a cave in the hills of Tabasco,

you can find a hint of what a living planet without oxygen miğht be like.

(WATER FLOWING)

This is one of the more unigue environments on our planet.

This cave is full of sulphur. You can see İit in the water,

you can see that milky colour in the lake that's flowing through the cave.

That's dissolved sulphur.

And it's coming from hydrogen sulphide gas,

the source of which is actually not entirely known.

The hydrogen sulphide is toxic to me.

And it has another, rather alarming effect on this hellhole.

İt's a bad-smelling gas, but it's also the gas that drives the oxygen out.

So, certainly as you go on into the cave system,

you get less and less oxygen.

So, in a sense, some of the chemistry and the biochemistry

that takes place in the dark in these cave systems

could be very similar to the chemistry and biochemistry

that occurred when our planet was very young.

For the first half of its history,

Farth was without oxygen in its atmosphere.

Bukt, incredibiy, in this echo of the past,

which I can only visit for a few minutes,

there are forms of life that are completely at home.

Wow, look at that.

There they are,

cities of sulphur-eating bacteria living off the hydrogen sulphide gas.

Colonies of extremophiles,

organisms that are living off a very different environment of gases

to the one that we're used to on the surface.

They are a window on a much earlier time.

Because without oxygen, the ancestors of these extremophiles

were the only forms of life our planet could support.

(WATER DRIPPING)

Ünderstanding how Earth developed an atmosphere rich in oxygen

has taken centuries.

And the secret ljes with ancient bacteria.

In 1676, a Dutchman called Antoine Leeuwenhoek,

was trying to find out why pepper İs spicy.

See, they thought that there were little spikes on peppercorns

that dug into your tongue.

He was using the microscope,

which had been discovered about 50 or 60 years before,

but inexplicably had never been used for anything useful before.

He put the peppercorns on there

and he looked down and he couldn't see anything.

So he thought, "Okay, I'll grind them up,

"dissolve them in water and have a look."

When he did that, he didn't see anything interesting in the peppercorns,

but he found that there were little animals swimming around.

And he said that he estimated you could line about

a hundred of the "wee little creatures", those were his words,

up along the length of the single coarse sand grain.

What Leeuwenhoek thought were animals,

were in all probability notanimals at all.

Although he didn't know it at the time,

he had discovered a whole new domain of life.

Bacteria.

They are by far the most numerous organisms on the EFarth.

In fact, there are more bacteria on our planet

than there are stars in the observable universe.

And there is öone kind of bacteria more numerous than all the rest.

One of the most striking structures I can see on this slide is a

kind of a blue-green filament which is a

little colony of a type of bacteria called cyanobacterla.

These things are incredibly important organisms.

Fossilised cyanobacteria have been found as far back as 3.5 billion years ago.

And at some point around 2.4 billion years ago,

they became the first living things to use pigments to split water apart

and use it to make food.

This evolutionary invention Was incredibly complex.

Even its name is a mouthful.

Oxygenic photosynthesis.

İt starts with a photon from the Sun hitting that green pigment, chlorophyll.

Chlorophyll takes that energy

and uses İit to boost electrons up a hill, if you like.

When they get to the top,

they cascade down a molecular waterfall

and the energy is used to make something called ATP,

which is essentially the energy currency of life.

This little molecular machine is called Photosystem 2,

and it makes energy for the cell from sunlight.

But when the electrons reach the bottom of that waterfall,

they enter Photosystem 1.

They meet some more chlorophyll which is hit by another photon from the Sun.

And that energy raises the electrons up again

and forces them onto carbon dioxide,

turning that carbon dioxide, eventually, into sugars, into food for the cell.

Now, why all this complexity?

Why do you need these two photosystems Joined together in this way

Just to get some electrons and make sugar and a bit of energy out ofit?

It's because only when life coupled these two biological machines together,

that it could split water apart and turn it into food.

But it wasn't easy.

The thing is that Water is extremely difficult to split.

So for aleaf to doit, for a blade of grassto doit,

jJust using a trickle of light from the Sun is extremely difficult.

In fact, the task is so complex

that, unlike flight or vision,

which had evolved separately many times during our history,

öxygenic photosynthesis has only evolved once.

Every tree, every plant, every blade of grass on the planet,

everything that carries out oxygenic photosynthesis today

does it in exactly the same way.

And the structures inside every leaf that do that

look remarkably similar to cyanobacteria.

In other words, the descendants of one cyanobacterium

that worked out, for some reason,

how to couple of those complex molecular machines together

in some primordial ocean, billions of years ago,

are still present on the Earth today.

The cyanobacteria changed the world,

turning it green.

And that had a wonderful conseguence.

With this new way of living,

life released oxygen into the atmosphere of our planet for the first time.

And in doing so over hundreds of millions of years,

it eventually completely transformed the face of our home.

And as the oxygen levels grew,

the stage was set for the arrival of ever more complex creatures.

But on Farth, the emergence of complex life

regğuired a rather more intangible ingredient.

Something that you can't see, touch or smell.

And yet you pass through every day.

Late January,

and the monarch butterflies have found their way home.

They've entered a hibernation state,

huddling together for warmth.

But they're only here at all

thanks to one of the most accurate biological clocks found in nature.

These are the pine and oyamel forests

at high altitude about, what, three hours north-west of Mexico City

and one of the few wintering grounds of the monarch butterflies, as you can see.

But there is a colony, millions of monarchs,

somewhere due north of here.

So if I head off into the forest,

then hopefully this will just be a taster of what's to come.

To find the butterflies, 1 need to get an accurate bearing on them.

And to do this, 1 need a timepiece.

If you don't have a compass,

how can you tell which direction's north and which direction's south?

Well, you can use the Sun.

The Sun rises in the east, sets in the west.

And at midday in the northern hemisphere it's due south.

But what ifit isn't midday?

Well, there's an old trick, which is to use a watch.

See, it's about 3:00 in the afternoon now,

and if you line the hour hand of your watch with the Sun,

then in the northern hemisphere a line in between the hour hand and 12:00

will point due south.

Which means that north, is that way.

For thousands of miles on their way here,

the monarchs have faced the same problem.

To make their way south, it's no good simply following the Sun,

because as the day progresses,

the Sun's position drifts across the sky.

Somehow, they have to correct for this.

They use what's called a time-compensated Sun compass.

They measure the position of the Sun every day using their eyes.

But it's also thought they can measure the position even when it's cloudy,

by using the polarisation of the light.

Having locked onto the Sun,

their brain then corrects for its movement across sky

by using one of nature's most accurate timepieces.

By combining the information from their precise clocks and their eyes,

they can navigate due south.

That ability to orientate themselves is,

Ithink, one of the most remarkable things I've seen.

The biological clocks that have brought the monarchs home

are not unigue to butterflies.

Almost all life shares in these circadian rhythms.

They're an evolutionary conseguence of living on a spinning rock.

Our world turns on its axis once every 24 hours, giving us a day.

İt's on a billion-kilometre journey around the Sun,

and each orbit gives us a year.

We live inside a celestial clock,

öne that has been ticking away for over four and a half billion years.

And that's a full third of the age of the universe.

(MEXICAN MUSIC PLAYING)

(HORSE NEIGHS)

This is the final ingredient that our home has provided.

Time.

Take the horse.

Like all complex living things,

it's here because our planet has been stable enough

for long enough to allow evolution time to play.

The horse is the animal whose family tree we know with the highest precision.

So it's possible to lay out Just öne unbroken chain of life

that stretches back nearly four billion years.

Animals that are recognisably horse-like have been around for a long time.

Something like 55 million years.

You then have to jump duite a lot to something like 225 million years

if you want to ask the guestion, where is the earliest mammal?

And it's this thing which looks something like a little shrew.

535 million.

This is the point when complex life really began to explode in the oceans.

You then have to sweep back a long, long time

to find the next evolutionary milestone,

arguabliy the most important milestone.

The emergence of the complex cell, the eukaryote.

Then, you have to step back a long way in time.

You have to step back

all the way to here,

the emergence of the prokaryote, the first life form.

And so we have this beautiful long line.

We can trace my friend, the horse, and his ancestry back

to events that happened 3.5, 3.6, 3.7 billion years ago

on the primordial Earth.

Now, looking back over that vast sweep of time

you could ask yourself the guestion, well,

do you need three and a half billion years

to go from a simple form oflife to something as complex as a horse?

Well, the answer to that guestion is we don't know for sure.

İt seems that you need vast expanses of time.

But do you need those big gaps from the simple cell to the complex cell?

Do you need the gap from the complex cell

to the evolution of the multi-cellular life?

We don't know.

We only have öne example.

There is only öone planet

where we've been able to study the evolution of life.

And it's this one.

And Farth has been an interesting mixture of stability and upheaval.

It's had an environment that's never completely conspired to wipe out life,

but it's constantly thrown it challenges.

The deep time that our planet has given life has allowed it to grow

from a tiny seed of genetic possibility

to the planet-wide web of complexity we're part of today.

Only a few of us have ever stepped outside of this world.

But those that have discovered something rather wonderful.

ASTRONAUT (ON RADIO): For all the people back on Farth,

the crew of Apollo 8 has a message that we would like to send to you.

On Christmas Eve, 1968, my first Christmas Eve,

the Apollo 8 spacecraft entered the darkness on the far side of the Moon.

ASTRONAUT: Zn the beginning, God created the heaven and the Farth,

and the Farth was without form.

COX: 7he three astronauts Borman, Lovell and Anders,

became the first human beings in history to lose sight of the Farth.

ASTRONAUT: Arıd God salid, "Llet there be light " and there was light.

And God saw the light, that it was good.

COX: When they emerged from the dark side of the Moon

and the Earth rose into view,

they chose to broadcast their culture's creation story

back to the inhabitants of Earth.

And just like the Aztecs and the Mayans and every civilisation before them,

it told of the origins of their home.

ASTRONAUT: Arrd God called the dry land Farth,

and the gathering together of the waters, called He seas.

And God saw that it was good.

İt must be innately human,

the desire to understand how our home came to be the way thatit is.

Seen from a lunar orbit against the blackness of space,

the Earth is a fragile world.

But seen by science, it's a world that's been crafted and shaped by life

over almost four billion years.

So we're on our way to understanding how we came to be here.

But as the Apollo astronauts discovered,

the journey of discovery has already delivered much more than just the facts,

because it's given us a powerful perspective

on the intricacy and beauty of our home.

ASTRONAUT: #rom the crew of Apollo 8,

we close with good night, good luck, a Merry Christmas,

and God bless all of you. All of you on the good Farth.